1. Field of the Invention
The present invention relates to a fluid bearing for supporting a rotary shaft by means of a pressurized fluid.
2. Description of the Prior Art
In general, a bearing device for a spindle of a machine tool is subjected to an excessive load, such as vibration load, during machining operation. A bearing device depending on only a static bearing support may not be able to bear such excessive load. For this reason, there has been required a fluid bearing capable of heightening bearing rigidity during rotation of the spindle.
In a conventional fluid bearing, a hydrodynamic pressure generating zone was formed on the bearing surface in addition to a hydrostatic pressure generating zone in order to increase the bearing rigidity during rotation of the spindle. A typical example was such that a land portion was formed within a fluid pocket. However, since the land portion was formed within the fluid pocket, it was difficult to enlarge the area of hydrodynamic pressure generating zone. It was also difficult to extremely decrease the diameter of a throttle formed on the fluid pocket fur supply of pressurized fluid in order to prevent the same from being clogged by a foreign substance, and to increase the amount of fluid flow from the fluid pocket. For these reasons, the clearance between the spindle and the bearing surface had to be made larger than that of the usual plain bearing in order to get a throttle ratio required for proper construction of the hydrostatic bearing which resulted in lowering the hydrodynamic effect.
In order to overcome this disadvantage, it has been considered to enclose the fluid pocket by exhaust grooves to increase the amount of fluid flow so that the clearance between the spindle and the bearing surface may be made smaller. However, according to this construction, the exhaust grooves communicating with atmosphere were formed axially of the bearing surface so that air was sucked into the bearing surface as the spindle was rotated, resulting in cavitation. In particular, since a plurality of pressure generating zones, each being constituted by the exhaust grooves and the land portion, were arranged in the circumferential direction, compressible fluid resulting from the mixture of air was supplied to the land portions. Accordingly, hydrodynamic pressure was not as high as expected and thus whirling of the spindle being rotated at high speed was produced.